The present invention relates to a light guide member. More particularly the invention relates to a light guide member for use in a read out apparatus wherein a stimulable phosphor sheet on which a radiation image has been stored is illuminated with stimulating rays which cause the phosphor to emit light in proportion to the amount of energy stored therein during its exposure to radiation, and wherein light emitted upon stimulation is guided by the light guide member from the phosphor to a light detector.
Light guide members that are suitable for the above application are known in the art.
A first type of such light guide members consists of a sheet of light transmitting material. In a read out apparatus one end of the sheet is placed adjacent to a scan line and the other end is located adjacent to the light receiving face of a photodetector.
The end of the light guiding member which is adjacent to the scan line is shaped into a linear form and the other end which is to be located adjacent to the light receiving face of a photodetector is shaped into an annular form to be adapted to the light receiving face of the photodetector.
A light guide member of the above-named kind has been described in U.S. Pat. No. 4,485,302. The described light guide member is made of a material that is transparent to light emitted by the phosphor. It is finished to have a smooth surface so that the light transmitted thereto is effectively subjected to total reflection by the internal surface.
Suitable materials which have adequate properties for light guiding are transparent thermoplastic resins such as acrylic resin, vinyl chloride resin, polycarbonate resin, polyester resin, epoxy resin or glass.
An alternative embodiment of such a light guide has been described in U.S. Pat. No. 5,352,903 and has two light input end faces being arranged so as to face the same scanning line. The first light end face is arranged for collecting light emitted from the front face of the stimulable phosphor sheet whereas the second light input face is arranged for collecting light emitted from the back surface of the phosphor sheet.
Portions of the light guide member which are adjacent to the output end faces are cut into a plurality of strips which are bundled, and end faces of the bundled strips are located in close contact with a detector, e.g. a photodetector.
In order to capture as much light as possible and to effectively guide the incoming light from the inlet towards the outlet of the light guide member, it is necessary that the probability of internal reflection is optimised and that loss of light inside the light guide member is reduced to a minimum.
In order to obtain these objectives, the shape of the sheet should be optimised and its surface should be smoothed. Preferably the light guide member should be made of a transparent sheet curled into a shape having a linear or arcuate inlet and an annular outlet.
Furthermore in order to enhance light transmission efficiency, the curvature of the sheet should be made small so as not to interrupt the repeated internal reflections.
Consequentially the length of the sheet should be long enough to allow the small curvature.
Another type of light guide members consists of a number of individual optical fibres that are arranged in a row at the side of the phosphor sheet to be scanned and bundled at the side of the photodetector.
The optical fibers used in this type of light guide are conventional optical fibers that consist of a core material surrounded by a solid cladding material having a lower refractive index than that of the core material.
Light emitted by a phosphor upon stimulation enters the light guide member and is guided inside the fibres by internal reflection towards the output end of the light guide member where the fibers are bundled e.g. to form a circular plane that can be coupled to the entrance window of a detector such as a photomultiplier.
Unfortunately the acceptance angle of these conventional optical fibers is only about 62 degrees equivalent to a numerical aperture equal to about 0.51. Although fibers having a higher numerical aperture exist, they all show higher attenuation in the near ultraviolet region. This is undesirable since storage phosphors screens frequently emit light in this spectral region.
Furthermore, when the light guiding trajectory is curved, the numerical aperture of the optical fibers decreases since even at moderate curvature the optical fibers are no longer able to guide marginal rays of light entering the input window of the light guide towards the output window.
Furthermore, due to inter fiber surface losses and due to losses at the entrance window of the light guide member because of the presence of cladding material, the light guide members consisting of optical fibers may lose about 15 to 20% of their optical efficiency.
It is an object of the present invention to provide a light guide that has a high optical efficiency. It is a further object to provide such a light guide for which the light guiding efficiency remains high even at a strong curvature of the light guide or part thereof.
The above mentioned objects are realised by a light guide having the specific features defined in the claims.
The new concept supports a light acceptance angle of 180 degrees equivalent to a numerical aperture equal to 1.
When compared with the prior art light guide concepts that are based on optical fiber assemblies, the concept of the present invention is thus advantageous in that a much higher numerical aperture can be attained. This higher numerical aperture remains constant over a wider curvature range of the light guiding member.
At a given numerical aperture a much stronger curvature of the light guide is still acceptable so that the apparatus wherein the light guide is placed can be made much more compact, as will be shown furtheron.
Depending on the application, the light-guide assembly can have a single or multiple input and output windows.
The spectrum of the light can cover areas such as:
Ultra-Violet, Visual and Infra-Red thereby supporting efficient transport of photons from narrow and wide band spectrum light-sources.
The transfer of light between input and output is done by a multitude of light-channels.
These channels have a solid structure and can be made out of any type of light-guiding material with a low degree of internal scattering and with a low absorption for the spectrum of the light it has to transport from input to output.
Possible channel materials are optical grade plastics such as polymethylmethacrylate (in the following referred to as PMMA), Poly Carbonate and Poly Styrene or they can be any member of a silica-based variety of optical glass materials.
Unlike optical fibers the light carrier (or light channel) itself does not have a solid cladding material with a lower refractive index surrounding the core material. Instead the light channel according to the present invention entirely consists of xe2x80x9ccorexe2x80x9d material.
These xe2x80x9ccorexe2x80x9d channels are suspended in a gas having a much lower refractive index than the core material. This concept ensures the highest possible numerical aperture even at relatively strong curvatures when compared with conventional fiber technology, as will be proved further on.
Light travels through the channels by means of consecutive total internal reflection at the glossy (microscopically quasi flat surface) and clean interface between the channel-material (the core) and the surrounding gas (the clad).
Ambient air or other gasses can be used as cladding gas.
The light-guide assembly can consist of channels with different cross-sections and cross-section shapes.
Circles, ellipses, squares, rectangles, hexagons or any other type of polygon or free shape can be used as the light-channels cross-section shape.
However, in order to preserve a 180 degrees acceptance angle, the individual light channels preferably have a constant or increasing cross-section along the channel""s three dimensional trajectory from input to output.
In a particular embodiment of the present invention, the individual light-channels are fused at the light input and/or light output sides by heat and by pressure to form core material blocks. These blocks preferably have the shape of the input and/or output windows of the light guide. The fusion-depth of the material block is preferably a few millimeters along the light-trajectory. Within the fused areas the boundaries of the individual channels have completely disappeared.
Since the blocks do not contain gaseous or other enclosures they are totally transparent and have a full 100% fill-factor at the window surfaces. This is especially important for the light-collection efficiency at the input-window.
The 100% hermetically sealed solid window-blocks may be polished for efficient light-coupling.
Since the blocks are hermetically sealed, the penetration of contamination due to capillarity in surface pits or cracks is avoided.
This is an advantage when compared to traditional light-guides using optical fibers. Such optical fiber assemblies may lose 15 to 20% of optical efficiency due to the inter-fiber surface losses and the cladding material surface losses at the input window.
In one embodiment the window blocks are suspended at the input and/or output windows by means of optical window plates. Preferably the same material as the core material is used for the window plates, alternatively they can be glass-based or plastic-based too.
They are preferably fused to the core material blocks.
Alternatively these plates can be glued to the core material blocks. Preferably index-matching optical cement is used for glueing in order to obtain minimal added reflections at the interfaces.
These window plates should preferably be spectrally suited to guide the light travelling through the light-guide.
A surrounding system is provided and arranged to hold the window plates in place.
An alternative but optically slightly less performing way of holding the fused and polished window blocks is to grab them at the sides by means of a tight-fitting frame-work of short reflectors. The reflectors are arranged along the trajectory path""s extremities.
These reflectors should preferably be specular and spectrally suited to reflect the light travelling in the light-guide.
Likewise a surrounding system can be provided to hold the reflectors in place.
The light-guide assembly is suspended in a surrounding gas (the xe2x80x9ccladxe2x80x9d material). It may be protected from contamination and from entrance of external light by means of a light-tight and dust-tight enclosure spanning the input and output windows. This cover is preferably not touching the individual light channels.
At the level of the interface with other system components the light-guide can be equipped with an optional anti-reflection coating or it can be coupled with other system components using optical oil mounts (removable) or optical cement (fixed).
An optional anti-reflex coating, the oil mount or the cement helps to reduce optical reflections at the light-guides boundaries and should preferably be spectrally suited to guide the light.
Preferred embodiments of the invention are disclosed in the dependent claims.